1. Field of Invention
This invention relates to a base station, a multi-user demodulator, and a demodulating method for the base station, which communicates with a plurality of user stations by DS-CDMA (Direct Sequence-Code Division Multiple Access) spectrum spreading communication method.
2. Related Art
In DS-CDMA spectrum spreading communication, a base station receiver receives signals from a plurality of users, separates and extracts each of individual user's signal from the received signals, accounting for interference among signals, and despreads the extracted signal. To despread the signal, a sliding correlator performs corrleation. The sliding correlator requires accurate phase-synchronization between the spreading code of the input signal and the spreading code of the receiver. Therefore, a synchronization detecting and tracking circuit, such as a DLL (Delay Locked Loop) circuit, is necessary. However, the DLL circuit has a disadvantage that the initial and subsequent leading times are long. Moreover, since it is difficult to synchronize accurately even with the DLL circuit, receiving characteristics are deteriorated.
To prevent interference among multiple user signals, interference cancellation is used. The interference canceler reproduces each user signal by despreading and recomposing the received signal, and by respreading its spectrum. The interference canceler further extracts a specific user's signal by subtracting other users' signals from the received signal. These processes required fairly large and expensive circuits.
Therefore it is an object of the present invention to overcome the above disadvantages, and to provide a multi-user demodulator and demodulation method that lacks the synchronization problems associated when there is no interference canceler. It is also an object of the present invention to provide a base station for communicating with a plurality of users using the demodulator.